Is your therapist a clinical psychologist or psychiatrist? If not, I would take what she says with a great skepticism.
> I feel sure that I did not have ADHD symptoms during childhood.
That would be an instant disqualification of an ADHD diagnosis.
> I believe have come extreme and unusual mental stresses I have been subjected for several years
I am not a clinician nor do I know the full extent/magnitude of your stresses, but have you ever investigated (C)PTSD? If can mimic many ADHD symptoms.
> However, there's not enough literature on ADHD coming during adulthood. . . Most, like the OP, say that it would have gone undetected during childhood. Some say that ADHD can develop during adulthood too. I have also read that Adult ADHD seemingly has a different mechanism than ADHD that develops during childhood.
Based on what is commonly understood about the condition, it cannot come on in Adulthood. Similar to how there is no Adult-onset Autism. You're born with it or without it. Also, just because symptoms were not "detected in childhood" does not mean the symptoms did not exist in childhood. My symptoms were not detected in childhood, but oh boy, were the signs and symptoms there. I just grew up in the South East, US where stuff like ADHD just didn't exist, and I and plenty others were punished and ridiculed for our "daydreaming," "restlessness," "laziness," and "inappropriate behavior."
Personally, I am not sure if I buy the research on this topic though. Then again, I also think the label "ADHD" is just like "Cancer" -- a massive blanket-term for multiple similar conditions. So, who knows? It's not like there is a single biomarker that has ever been discovered that can be used to diagnosis the condition. It's all just professional opinions based on agreed upon patterns and heuristics.
Anyway, I hope you figure out something that works for you.